A corpobation of dela



May 22, 1928.

- F. JEFFREY mnuc'rion MOTOR CONTROL SYSTEM Filed May 29 1 2 Patented May 22, 1928.

UNITED STATES 1,671,076 ,ATEN JT. lorries.

FRASER ,mrrnnr'or MILWAUKEE, Wisconsin, *iissrivonroamrs cmnmnns Mair- UIEACTURING COMPANY, or MILWAUKEE, wrsconsmq n conrona'rron or DELA-' Application filed May 29,

.flhis invention relates in general to the speed controlof induction anotors, and has particular relation to the speed controlfof induction motors where the control 'is exertofthe armature; of a,dire ct:currentf machine adapted ,to supply energy to and-absorb en ergy froinfthe alternating current system in which the induction motor is connected.- Such a system ofucontrol is capableof producing-.satisfactorilystable operation of the motor at:- difierentf speeds-below synchronisin,- ,and this without-the heavy losses pres'-. ent where E the: loiver speeds are (secured: qbi

insertingaresistance z in-the secondary. circuit mIOtOr. 4

1 With ;thi s Kraeiner system of; control, the speed-is yvaried by varying the excitation of the fieldof. the..directcurrent machine. .1111 theory, :a set -.off. this" character .lis justns' readily operableat any-'speedg-abovel syn chronisn'i as below synchronism,- .the'variation ini-spedfrom the minimunrsubsyn chronous speed being effected by the. reduc-- tion of the field of the direct currentmmorto zero, at which time, according to-the=the-;

or tliezspeed -df the set, should be the synchronous speed of the induction motor, and to bring the speed above synchronisin should be merely'la matter of reversing and increasing the'field on a direct current machine. Howeven'due, probably in the greater part, inthe. presence of considerable inductioniu the circuit lwhich includes the 1 secondary Jot the induction-motor through'the rotary-converter and the direct'current machineQ e'ven withno loadvo'n theinductionfinotoi', it is extremely; -.ditlicult-= and impracticable- -to' bring the set througlractual synchronism and to a stable operi-iting speed above :syn-

chronism.- .Thisuis the caseto awgreater ex- --"It is an object-of this invention to provide an improved method of controlling induction -mot-ors,i and an improved system of tent-when thereis a ?load upon; the induction mnncrioN-moron-conrnor. SYSTEM."

192o.---se'ria1-No. 385,734.

.r. {"z'fll ")i'i J i control-. or induction motors in accordance with .-whicli' the :speed is controlled through aurotary converter and adirect current machineandE-the set is operable atanydesired speed. below or above synchronous speed of en uct o niQ Q I-w-Z z: ;f;' $-%l1d-9th l'ipble ian i advantages are attained by this invention,j var-i0 us novel feation and; drawing forming part of this ap plicatioir; and disclosing an: embodiment of the invention; '.will ;be inore particularly pai edout ntth fcla m .1;

The drawing gis a- ;.diagran Lma-tic showing of a system of, induction motor control embodying' features of this invention.

According to the, illustrative; disclosure, an induction; notor; 1 0 has ,its secondarywinding of 'thephase-wound type, and connected througlrsliprin gs-B and a'switch 1st to; a yariable; resistance ;13 and has. its, primary, circuit qonnected through a reversing switch 15, .to, a supplypircuit 16. A rotary converter 17 ha s ;its. alternating currents'ide connected;through-slip rings 18 and a switch 19,; to the, slipirings; l2 of the: induction m0- tor 10,; The direchcurrent field 21' of the ,ro-; tary converter .17 .isv;connected to a .direct curi'ent-j'supRlyline 23 through a switch 24, a variable resisftance 25 in theform of a rheostat beingincludediin the. gficld circuit. The direct current-:side 26 0f the rotary converter 17 is connected to the ar'nnitureof the direct current machine 28 the armature being here shown as mounted upon the shaft ofthe induction motor'lO. The connection between the 'rotaryconverter and the direct current machine is through a switch 29.

The field 31,0f the direct current machine is supplied from the direct current source 23 through a switch 32, 11 variable resistance 34, preferably inLthe. form of a rheostat, being provided in the-field circuit, An auxiliary connection between the field 31"and the circuit 23 is providedthrough a switch 35, the connections being such that closure of the field circuit through the switch 35 has the effect of completing this circuit in a direction opposite to that exist-ing'when the circuit is closed through'the'switch 32. A variable resistance 36, preferably-in theform of-a rheostat, is provided in the circuit of the switch 35. An automatically operable variable resistance 37 is a'lso provided in the circuit of the field 31 closed through the switch 35 and the rhcostat 36. An auxiliary connection is provided between the supply circuit 23 and the armature of the directcurrent machine 28, this connection being completed. through a switch 38, an automatically operable variable resistance 39 being included in this circuit.

As indicated in the illustrative disclosure, the control of the various switches is preferably through operating electromagnets. the circuits oi? the magnets being energized i'n anyide sired selective fashion, as through thedrumcontrollers 47 and 4.8,shown diagrammatically. All of 'the controlling switches are biased to open position, as by means of springs, as indicated in the drawin'g. Certain of the switches interlock with others to insure that but one of the interlocked switches is closed at any instant. The energizing circuits of the operating elect-romagnets are completed from the direct current supply line 23 through the controller drums 47 and 48 in such a'manner as to secure'thc desired sequence of operations.

The reversing switch in the primary circuit of the motor 10 is closed to cause forward operation of the motor by the operating magnet 51, and is closed in a reverse direction by operating magnet 52. The arrangement of the operating magnets and the spring biasing means are such that the switch-is returned to'open position when both-of the electromagnets are de-energized, as indicatedin the drawing.

The switch 14 is actuated by the electro magnet 53, and the switch 19 by the electromagnet 54, the electromagnets 53 and 54 being mechanically related so thatoperation of one of the switches to closed position prevents closure of the other switch at the same time. The interlocking means may be anything desired, the rna ns shown including a mechanical connection between the operating means for the two switches and a double-acting spring means effective to re turn both of the switches to open position when both magnets are de -energized. The switch 24 is actuated by the operating magnet- 56, The switch 29 is actuated by the. electromagnet- 57, and the switch 38 by the electromagnet 58, the switches 29 and 38 and their operating means being so related as to insure the closure of only one of these switches at the same time. The switch 32 is actuated by the electromagnet 59, and the switch 35 by the electromagnet 61, the two switches and their opera-tingmeans being interrelated in substantially the same man-' ner as the pairs of switches described above, so asto insure closure'of only one of the interrelated switches. l

The operating means for the variable-resistance 13 in the secondary circuit of the inductionmotor 10 is of a type suitable to secure the autou'latic cutting-out of the re sislance as the speed 0t the-motor increases during the starting operation, the arrangementbeing designed to prevent-excessive current in the circuit of the motor insun m the proper ree ofa1 leration ot the mam-J 'lhet yp e of automaticregulator shown in connection with the resistaiice 13 includes an electromagnetic actuating device comprising a core and its winding supplied. from the circuit 23 through the controller drum 48, this winding tendingtio op crate the regulatorin a'd i're'ct-ion to cut out resistance, and a secondcore and its winding 67 in series with the secondary circuit of the motor 10, the latter element 'ofthe regulator'exerting an effect opposite to' that of the elcctromagnet 65.= The regulator is biased by gravity, as indicated, "or other suitable means, to the'p0s'ition that includes maximum resistance in the secondary *cirflcuit, During starting of the motor, the c-lectromagnet 65 is effective to actuate the regulator only as-fast as the series winding of theelectromagnet 67 will permit, this speed of operation being predetermined so as to provide for a. safe -currentinfthe secondary circuit of the motor The automatic regulator for the variable resistance 39 is of substantially the same type as that-described inconnection with the variable resistance 13 in the secondary circuit of the motor 10, and includes an elc tromagnet '68 sup-plied fmm the source 23 through the controller drum 48, and an elect-romagnet 69 having its winding 'in-series with the resistance 39 in the' armature circuit of the direct current machine 28, through the switch 38. The regulator is biased by gravity, as indida-ted,or other suitable means, to the position which ineludes maximum resistance in cireuitf The energizing efiects of the electromagnets 68 and 69 on the regulator are opposite, the general design'heing such that the magnet 68 is'efl'ective' to operate thewegulator to cut out resistance only as fast as the series winding ofthe magnet 69' permits, this rate being predetermined from the safejratc of acceleration of the machine 28, when the latter is operated as a motor from the supply line 23, as will be described hereinafter.

The automatic regulator for the variable resistance 37 includesan electromagnet 71 supplied from the circuit-'23. through the controller drum 48, and an electromagnet- 72 having its winding in series with th'earmature circuit of the machine 28, which is completed through'the switch -38. This automatic regulator is biased by gravity, as indicated, or other suitable means, to' a "position wherein the resistance 37 is cut out of circuit,'energization of theseries magnet "72 tending to holdthe regulator in this'posif tion. Energization of the 'electromag'net =71 is'effective to cause the regulator to insert the resistance 37- at a predetermined rate depending upon the counter E. M. F. of the machine 28 as measured by the current in the armature circuit in which the winding of magnet 72 is included.

In starting the-induction motor, for operation thereof at a speed below synchronism, controller 47 is moved to position No. 1, wherein the circuit of the winding 53 is closed, causing closure of the switch 14. In position No. 2 of this controller, theswitch 15 is closed in the forward direction through energization of the electromagnet 51; Likewise, the'electromagnct is energized and the automatic regulator is effective tocut out the resistance 13 a speed determined by the current llIl tlle serics inagnet 67', thus permit the-motor to 'beb'rouglitiip to speed at the predetermined rate1f"The niaxiinum motor spe'ed'with these connections: isslightly below synchronism. I I

Tooperate the set at a vai-iable'spe'edbelow synchronism, the controller 47is moved to ptis ititir No, 3', wherein electromagnet 65 is d'e-eiler gized, electroinagnet 53 is de-energized and 'electromagnet 54 is energized, causing"op ei iing o'fjthe switch 1 and closure of the switch 19', thus connecting-the 'secondary of the 'motor '1 0 to the} alternating current side of the rotary convlerter 17, and electromagn'et is energized "to-cause closure of tliejfie ld' switch 24 of the rotary converter '17 connectin'g'jtl ie' field circuit to the 1s'ene rgizedfcausing closure of the switch 29 to connectthe rotary 'converter'to the directcurrenti' machine 28, and electromagnet 59 is energizedmausing closure of the's'witch 32 to connect'the fieldieircuit 31 o'fthe direct current machine 28to the "supply line 23.

With the connections completed in position No. 3 of 'the controller, the" speed of the induction motor is dependent upon the amount of resistance in the circuitof the field .31 of the direct current. machine 28, and the powerfactor of the systen'i is dependent upon the amount. of resistance in the field circuit 21 of the rotary converter 17. It will be apparent that with the above connections, the speed of the set can readily be varied from a'speed approximating synchronism to a lower speed'determined by the size of the rotary converter and the direct current macliinerelati e to the induction motor 10. a In case it is desired to operate the motor at speeds above synchronism, 'the required connections mightbe'made through the controller 47, but for purposesof present convenience these connections are made through a drum controller 48. I Assume that the induction motor is at I rest. Movement of the n lerjfl wjpo i fl. =NQ- causes energization'of"the' 'electro'mag'n'et' 61 and consequent closure ofthe switch 35, thus completing the field circuit 31' of the direct current machine for current flow in a direction opposite to that in which current fiows'when this'circuit is completed through the switch Under these conditions, the rheostat 36 and the automatically. operated rheostat 37 are set to allow full field on the motor. Movement of the controller to position N o. 2

causes energizationofthe magnet 58 and consequent closing of the switch 38, complet ing'the 'arr'r'iatu-re'ieireiiit of the direct current machinefzs froi n' tli e' circuit 23. The electroniagiiet- G8 is *also energized, the effectof energization stiles-magnet being to tend "to move the regulator for the rar'i-- able resistan'cr-i'39 in such a directi'o'nf as to cutout resistance; but the resistance-decreasi ng' 'operation o'f 'the regu'lator is hindered b3} -'th&' e1iaet'r'emagnec69{whqsewindin is in series with the armature ofth'e machine 28, a; such an extent as to" permit acceleration of the machine '28Ia't a, safe rate. \Vhen the resistance 39*liasibeen entirely-cut out, the rheos'tat 36 ar a be operatedto insert res'is tan c'e "iri the ,ifiel-d" 'c ircuithto 4 further ncrease the 's'peedfoft the machine 28."

In position No; 3' of the cont-roller 48, the electromagnetlfl -is "energizedand tends to out out the" enabler-sesame 37. beingopposed byithe el'ecti'ditiagiit 72 whose winding is ins eries in'the' ariiiftiire circuit of the inaChine ZSL the resistan'cadcregsing opera t-ion occurring [at a 'rate that insures the required'de ree of acceleration of the machine 28 at the-ii the automatically-fregulatedresistance 'be so designed as be 'efiecti ife to the speed of 'tliei dir'e'ct currient'l r'nachine 28 "and the parts associated 1therewitlif i1 p to a predeter= minedj speed somewhat-above s' \j"ii'chronism, with substantially "all or -the greater part of the rheostat"resistance '36 included in the field circuit. I i On movement' of the"controller to position No. t the" remaiiider' 'of the "variable resistance 37 is cut into "circuit, in case such operationhas not been completed in position No. 3 of the controller; and the electromagnet 51 is energized to cause closure of the switch 15 in forward direction, the electromagnet 54 is energized to cause closure of: switch. 19, the elect'romagnet 56 is energized to cause closure of the switch 24 and the electromagnet 57 is energized and the electromagnet 58 de-energized to cause closure of the switch 29 and'opening of the switch 38. At the same timefthe' electromagnet 68 is de-energized, permitting the automatic regulator for the. variable resistance 39 to drop down to its normal resistance-in position. With these connections made, the speed of t-he'set eanhea'dily be varied by variation pfithe iresistance 36 in th'e field circuit 31 of themachine 28 Likewise, "the igi aris'peus. Itis advisable that P wer factor-of .the system may readily be varied by variation of the resistance25 in the circuit of field 21 of the rotary converter. 1

As another feature of this invention in the matter of bringingthe speed of the induction motor 10 to synchronous speed and sufficiently above the same to warrant stable operation of the motor at any super-synchronous speed, it is pointed out that when. the motor lO is operating fairlyclose to synchronous speed, the energy of the secondary circuit .-is hardly suflicient to producerotation of the rotary converter 17, or, at most, only rotation at a very slow speed. It now the; switching arrangements. aiesuch as to permit the ap )lication of direct. purrent of a. variable .voltage to the armature. of the converter and thefield of ,the converter is in the proper. direction, the converter will operate; inverted. .The; first effect, in case the, converter is actually, stopped, \v ill be he Pasa ilre c rr t. r u th rotary ainatnre and sliprings, this energy exciting the. secondary. of the indnction motor pullin thesame up to actualsynchronous spee In case the converter is not actually at rest, the application oi stiflicient voltage tothedirectcnrrent side ofithe converter will. produce. :sulficient counter w ifl ifi r e h vqnre t a mat re to rest,- and .at appreaimately this .Lt'nne sufficient direct current excitation. of. thelscc- Olilditllif of. the motqrfldwill cause fth'e latter to into synchronisni. The continued application ;of direct current. voltage to the converter will cause .the latter 'toirotate in the opposite direction, producing an alter, nating current: at theg'slip rings, this. cur} rent beingreversed phasefwith respect to the current that flows in this circuit when the motor 10. is operating at a.si ib-synch'r onous speed. With these conditions the speed of the motor 10 builds up above synchro nism to a value dependent upon the frequency of the current at theslip rings of the converter. Thearmature of the converter and the direct current machine 28, in case the latter is used to assist .in bringing the rotor of the motor 10 above synchronous speed, may now be disconnected from the circuit 23 and the converter and the armature of the machine 28 connected together. directly, in case this connection is not already made. With the direction of current flow in the field of, the machine 28 reversed from that existing du ri ng sub-synchronousoperation, it will be apparent that any? desired super-synchronous speed of operation of the motor 10 may be obtained by suitably adjusting the excitation of. the field of themachine i In order to obtain. substantially the Same eifect as thatde'scribed in connection with reversing the field 31 of 'tlieydii'ect current machine 28 during the operation the controller 48 described above, the direction of current fiow through the field may remain the same as for operation at sub-synchronous speeds and one phase of the circuit between the induction motor 10 and the rotary converter 17 may be reversed.

In order to secure quick stopping .of the induction motor 10, both of the controllers 47 and 48 may be provided with a single reverse position, indicated as R. then-the controller is moved from a running position through off position, to .thcposition R, indicating reverse, the connections made through the controllerare such that the electromagnet 52 is energized, causingclosure of the;

switch 15 in reverse direction, electromagnet 53. is energized and electroinagnet, 54 de} energized, cansing. closure of switch 14 and opening of switch 19, and electromagnet 6,5. is energized, The current in-the secondary winding of the induction motor is immediately increased due to the fact that itha s impressed thereoi' a voltage greater than the normal primary voltage. Thisin creased current actsas a load upon the. machine 1.0 and serves as a brake to decrease the speed of the machine. As the speed of'the machine and the. current inthe'secondary wind ing decrease, thei electromagnet 67 whose winding is'inseries with the secondarycircuit oft-he motor actjsjt o insure the presence of onlysuflicient res'istance in the secondary circuit to maintain the load on the induction machine at a maximum safe value' during slowing down, thus insuring stoppage of the motor in a minimum time. i

an uxi y 'Q subst tu m a fo driving the, rotor of the induction 'motor 10 at a speedabove synchronism'tojperniit op eration of the motor under its own power at speeds. above synchronous speed, an induction motor 76- may be provided. This induction motor has a number of poles less than that of the induction motor 10, and hence, a synchronous speed higher than that of. the motor 10. The primary circuit of the motor 76 is supplied from the supply circuit 10 through a switch 78, and the secondary winding of this motor isconnected through its slip rings 79 and the switch 81, to a variable resistance 82.

As has been described in connection with the other. apparatus of the unit, the switches '18 and 81 and an automatic regulator for the variable resistance 82 arepreferably actuated by. electromagnctscontrolled from a drum controller or other means. The switch 78 is actuated by an electromagnet 83, and the switch 81-h) an electromagnet S4. The automatic regulator for the variable resistance 82 is controlled by an electromagnetic actuating device comprising an electromagnet 85 whose winding is 'completed'throngh the controller, this latter electromagnet tend;

ing to shift the regulator in such a direction as to cut out resistance, and an elcctromagnet 86 whose winding isin series with the secondary circuit of the motor and tends to prevent shifting of the regulator by the electromagnet except when current conditions in the secondary winding are such as to warrant cutting out of the resistance 82. This automatic regulator is biased by gravity, as indicated, or other suitable means, to the positionthat includes maximum resistance in the secondary circuit; and it is generally similar to that described 'for controlling the variable resistance 13in the secondary circuit of the induction motor 10. A separate controller -87 is shown for controlling operation-.oftheinduction motor 76; but it will be apparent-that one or the other of the controllers 47, and :48 might-be modified to include means for securing the desired controL ofithis motor. i 1

It willbe apparentthat the motor '76 may be of such size relative to the induction mo- -tojr 10 andithe direct current machine 28 that the motor 76 alone may be operative to bring the-shaft 'ofthe. motor 10, and probably any ordinary load thereon, to the required speed above synchronous speed of the motor 10; and'thereawould be no necessity, under these circumstances, of operating the direct current machine'zu28 as a m'otorto bring the speed-of the induction motor 10; above syn ro'nism.' On the other hand, theinduction mot'or 76may -be'of a comparatively small size, invcase itisintended for operation in conjunction with the direct current machine 28 for bringing thespeedof the set above synchronous speed of the induction motor 10.

. For-purposes of convenience, assume that induction'motor 76 alone is used to bring the induction motor'lO to a speed above synchronism. On movement of the controller 87 to position N o. 1, electromagnets 83 and 84-are energized to cause closure of the switch 78 in the primary circuit and the switch 81 in the secondary circuit, respectively, of the motor 76; and electromagnet 85 is energized at the same time. With these connections made, it will be apparent that the machine 76 is automatically accelerated. the rate/of acceleration being determined by the effect of the series electromagnet 86 acting in opposition to the electromagnet 85, on the variableresistanc-e 82.

Under ordinary conditions. the speed of the motor 76 will, through the action of the automaticregulator in the secondary circuit, rise-to a point somewhat slightly below synchronismf for this motor. This speed attained by the motor 76 should be appreciably above synchronous speed of the lnduction motor 10; and when the desired speed has been reached, the controller 87 may be moved to position No. 2, in which position the electromagnets 83, 84and 85am de-energized, causing openingof the switches 78 and 81 and de-energization of t e electromagnet 85 of the automatic regulatorin the secondary circuit. In this position of the controllcr, elcctromagnets 51, 54, '56, 57 and (31 are energized to cause closure of switch 15 in a forward direction, switch 19, switch 24, switch 29 and switch 35 respectively. At the same time, the electromagnet7l of the automatic regulator for variable resistance isenergized, 'a suitablevamount of resist- ,current machine 28. The power factor of the system is varied asdesired by variation of the resistance25 in. the field circuit21 of the rotaryconverter. a

If, instead ofusing the inductio nmotor 76 alone to bring the set above synchronous speed of the induction motor l0, the motor 76 is used in conjunction f with the direct current machine '28, the desired operation of the system forincreasing the speed of the shaft of the motor .10 up to the .desired value is secured by moving controller 48 through position No. 1, to position No. 2, and,, at the same time, moving controller 87 to position N0. 1. With these latter connections, both the induction motor 76 and direct current machine 28 are free to exercise their motor effects in speeding up the shaft of the induction motor 10. It will beapparent that the desired connections for operating the system above synchronous speed may be secured by operating the con troller 48 to position No. 4, andicontr oller 87 to position No. 2. I i

As a safety ,means for insuring themaking of the proper switch connections, it may be desirable to interlockithe controllers 47 and 48 so that, in case either is inan Y position other thanhofi' position,.,t he other controller cannot be moved .from off position. Interlocks of the general type required are often used ,in connectionrwith a plurality of controller drums and, accordingly, there is no] necessity for any detailed description thereoi. 1

It will be apparent that set can readily:

be brought from a sub-synchronous operating speed to a super-synchronous speed, provided the loader] the set is'not beyond the P r f th m c ne 283 191 7 i ior fit in conjunction, by manipulation of the controllers 48 and 87 in the proper manner, as described above.

It should be understood that it is not desired that the invention claimed be limited 'to the exact details of construction and operation shown and described, for obvious modifications will occur to one skilled in the art.

It is claimed and desired to secure by Letters Patent:

1. In combination, an induction motor having a phase-wound rotatable secondary, and means for controlling th'ejsp'eed of said motorco'mprising a rotary converter Whose alternating current side is connect-ible to the secondary of said induction 'motor', a direct current machine adapted to operate either as a motor or a generator, a separate source of electrical power connectible circuit with one of s'aidrnac'hin'es and'adapted to insure 'an increase in the operating speed of the rotatable element of said induction motor from a sub-synchronous to a super-synchronous value, and controlling devices for selectively establishing an operative condition wherein said direct current machine is connect-ed to the direct current side of said rotary converter for operation of said in duction motor under normal conditions of *sub s'ynchronous operation or causing said separate source to supply energy required to insure raising the speed of the rotatable element of said induction motor from a subs'ynchronous to a s'uper synchr'onous value.

2. In combination, an induction motor having a phase-wound secondary, and means for controlling the speed of said motor comprising a rotary converter whose alternating currentside is connectible to the secondary of said induction motor, a direct current machine adapted to operate either as a motor or a generator, the commutator of said direct current machine being connectibleto the direct current side of said rotary converter, a separate source of direct current energy conncctible to the commutator of one of said commutator-type machines and capable of causing the supply of'energy through said machine to the secondary of said induction motor to cause operation of the latter at 21 Speed above synchronous speed, and means for selectively connecting the commutator of said direct current machine to the commutator of said rotary converter' for obtaining normal operating speeds of said induction motor or connecting said separate source of direct current energyto one of said commutator-type machines for raising the speed of saidiiiduction motor from a. sub-synchronous to a super-synchronous value.

3.- In combination, an induction motor having a phase-wound rotatable secondar element, and means for controlling the speed above its synchronous speed, and means for selectively 'conne'c'tin'g the armatureot' said direct current machine to the direct current side 'of said rotary-converter for obtaining normal operating-speeds'of said induction motor or to-said separate source of direct current energy for causing operation of said direct currentmachine as a =inot'0'r for raising the speed of said induction motor-from a 1sub-synchronous to a super-synchronous va ue.

4. In combination, an induction motor having a phase-wound secondary, and meansfor controlling the speed of said motor=to secure operation thereof at speeds: below :and above synchronous speed, said controlling means comprising a rotary converter, a directcurrent machine mechanically connected to the rotor of said induction motor and adapted tooperate either as a motor=or generato'r the alternating current side of said converter being electrically connectible to the secondary of'said induction motor, a separate source of electrical energy,- and means for selectively connecting the armature of said direct current machine to the direct current side of said rotary-converter or to said separate source of electrical energy for operation of saidmachi-ne as a motorf'or purposes of carrying the sp eed of said induction motor from a sub-synchronous value to a stable super synchronous value. I

5. In combination, an induction motor having a phase-Wound rotatable secondary element, and means for controlling the speed of said motor,"said means comprising a device capable of converting alternating current and vice versa, the alternating current side of said converting device being connectible to the secondary of said induction motor, a direct current machine adapted to operate either as a motor or generator, an auxiliary source of electrical energy, and means for selectively causing the operation of said induction 'motor at a-sub-synchronous or super-synchronous speed with the secondary of said induction motor connected to the alternating current side otsaid converting device and the direct current side of the latter connected to the armature of said direct current machine or causing said auxiliary source of electrical energy to raise the speed of said induction motor from a sub-synchronous Value to a stable snper-syncln'onous operating value through the supply of electrical energy from said auxiliary source to a portion of that circuit which normally includes said induction motor, said converting device and said direct current machine.

6. The method of producing op ration of the induction.motor of a Kraemer unit at stable super-synchronous operating speeds, which comprises supplying electrical energy from a separate source to the directcurrent machine of said Kraemer unit to insure bringing the speed of said induction motor from a snb-synchronous value through synchronous speed and up to a, stable super-synchronous value, and adjusting the field excitation of said direct current machine with its armature connected to the direct current side of the converter of said Kraemer unit and the alternating current side of said converter connected to the secondary of said induction motor to secure the desired stable super-synchronous operating speed thereof.

In testimony whereof, the signature of the inventor is affixed hereto.

FRASER JEFFREY. 

